UMICH-2015: Dark Energy / Gravity / Dark Matter break-out session 1

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Key Science Questions:

What CMB-S4 observations can be used to distinguish mechanisms of cosmic acceleration?

OR, should the key science question be along the lines: What observations can be used to falsify the default assumption of a cosmological constant?

What are the complementary probes?

Topics for Discussion:


  • Dark energy equation of state, sound speed
  • Early dark energy
  • General scalar theories (Horndeski, beyond Horndeski, modified gravity)
  • Focus only on scalar sector? What about vectors, tensors? - EL: in Horndeski, scalar sector linked to tensor sector so complementarity between CMB-S4 and galaxy surveys (DESI, LSST). Tensor propagation (low ell B-modes) related to gravitational slip of galaxy lensing/clustering.
  • Novel effects in specific theories
  • What models or figures of merit to forecast for S4?


  • Lensing, vs. other measures of mass - EL: CMB lensing sensitive to scale dependent physics, e.g. modified gravity Compton wavelength, dark energy sound horizon, neutrino free streaming scale.
  • Growth of structure (lensing power spectrum, cluster abundance and pink elephants, cluster profiles etc)
  • Velocity fields (RSD, kSZ)
  • ISW and cross correlations

Discussion: General Question: What are the unique opportunities for testing the physics of cosmic acceleration with CMB-S4?

We may consider that temperature, polarization, large-scale lensing will be well measured. Then what are the remaining cosmic acceleration science drivers?

If there does exist a new dark field, be it dark energy or part of some new gravitational phenomena, then we want to know how it interacts with the rest of the world.

Discussed in this session and others: Growth of structure, velocity fields, ISW and cross-correlations.

There is a unique window in scales and times that complement other dark energy probes. We need a few push-pins in this map to indicate reference theoretical models.

Refer to Euclid document (Amendola et al) that surveyed dark energy and modified gravity models and identified the observational impact of each.


Timeline for complementarity with other dark energy surveys


Unique CMB-S4 windows on linear regime, large scales and high redshift


CMB S4 forecasts for pairwise kSZ statistic for modified gravity FoMs